1. Field of the invention
The present invention relates to a method of forming an undoped silicate glass (USG) layer on a semiconductor wafer, and more particularly, to a method of forming an undoped silicate glass layer on a semiconductor wafer using high-density plasma chemical vapor deposition (HDPCVD).
2. Description of the Prior Art
Prior to metallization, dielectric material must be deposited onto the surface of a semiconductor wafer to isolate the MOS transistors and the metal interconnects subsequently produced. Generally speaking, the deposited dielectric material comprises a thinner undoped silicate glass (USG) layer for protecting and isolating the elements on the semiconductor wafer and a thicker planarization dielectric layer deposited over the USG layer as the main dielectric layer for subsequent metallization. The HDPCVD process is very effective in filling holes and is used to deposit the USG layer. The HDPCVD chamber is connected to the gas transporting system, the plasma generator, and the process control system. The reaction gases and plasma are transported to the deposition chamber then reach the surface of the semiconductor wafer to form solid material on the surface by chemical deposition.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of the deposit chamber 10 of HDPCVD according to the prior art. FIG. 2 is a top view of the injector 16 and the semiconductor wafer 14 shown in FIG. 1. The HDPCVD deposition chamber 10 comprises a pedestal 12 for supporting a semiconductor wafer 14 and a plurality of injectors 16 evenly placed on the chamber wall around the periphery of the pedestal 12. Reaction gases are injected in by the injectors 16, then used in processing on the surface of the semiconductor wafer 14.
When USG deposition is performed in the deposition chamber 10, the semiconductor wafer 14 is placed on the pedestal 12 first, then each injector 16 injects a mixed gas of argon, oxygen, and silane. This mixed gas will be ionized by the plasma and then diffuse onto the surface of the semiconductor wafer 14 to form a USG layer. By the nature of HDPCVD, sputtering and etching are simultaneously taken place and it leads to the ion bombardment on the wafer. This is the basic concept behind in-situ deposition and sputtering in HDPCVD process.
Generally, when performing USG deposition in the deposition chamber 10, injectors 16 measuring 4 inches in length are utilized. The opening of each injector 16 is above the area 18 of the surface of the semiconductor wafer 14 as shown in FIG. 2. Because the opening of each injector 16 is above the area 18, most of the mixed gases collect in the area 18 of the surface of the semiconductor wafer 14 during the HDPCVD process. Therefore, the USG layer deposited on the semiconductor wafer 14 will not be uniform with the USG layer within the area 18 being thicker than the USG layer outside the area 18. The USG layer in the center of the semiconductor wafer 14 will be thinnest of all. Usually this non-uniformity of thickness is the biggest drawback of the HDPCVD process causing problems in subsequent processing and greatly affecting the function of the dielectric layer.